Measured electric potentials, for example, on the skin of a patient and the useful signal contained in these potentials lies only in the mV range, as may be the case, for example, in an electrocardiogram (ECG) or an electromyogram (EMG), the following problems arise.
Since the body of the patient is surrounded by electric fields, potentials are formed due to capacitive coupling on the skin of the patient. This effect can generally be described in such a way that the body is coupled capacitively especially to a 230V/50 Hz alternating voltage field (it would be 60 Hz in the USA) which is caused by power supply sources located in the surrounding area of the patient. For the sake of safety, it is, however, not allowable to couple the patient himself to a uniform surrounding ground, because this would cause a considerable risk to the patient.
In addition, likewise for the sake of safety, a measuring device, to which the electrodes on the skin of the patient are connected, must also be galvanically separated from a surrounding ground. This in turn implies that the measuring device is also coupled with its internal ground capacitively to the surrounding area, so that the problem arises that the device ground lies on a potential, whose level is not known, and which generally differs from the potential of the patient.
In order to now at least achieve that the patient and the ground of the measuring device lie on the same potential or at least a fixed potential difference is present between both, it is known to connect the device ground and the body of the patient to one another via an additional electrode.
Since, however, the device ground and the patient may generally lie on a different potential because of the inhomogeneity of the surrounding fields, which arises from the different capacitive coupling to the surrounding area, an equalizing current flows, which leads to a so-called common mode signal because of the impedance of the coupling to the patient via the additional electrode, which is amplified by the amplifiers in the measuring device. When the useful signals actually to be detected with the measurement are very small, the common mode signal leads to the actual useful signal no longer being able to be resolved. Moreover, the difficulty arises that the amplifiers must have a high input dynamic range, so that the useful signal and the higher common mode signal overlaying this can be processed. Furthermore, a digital electronic analyzing unit arranged downstream has to provide a high number of bits per measured value to be able to process the large signals.
For this purpose, it is known from Bruce B. Winter et al., Driven-Right-Leg Circuit Design, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983, to apply a potential, which corresponds to the mean value of the signals detected at the measuring electrodes, wherein this mean value signal is also amplified in inverted form, i.e., a negative back coupling is present, to the additional, so-called common or additional electrode arranged on the patient by the measuring device.
It has now been shown that the quality of the signals detected at the measuring electrodes depends highly on how well the contact is between the additional electrode and the skin of the patient. Already when the electrode is easily detached from the skin of the patient, a markedly increased noise occurs in the measured signals or these signals are frequently unusable. Especially when a potential difference is present between the patient and the measuring device and an equalizing current flows over the additional electrode, there is a drop in voltage at the additional electrode/skin contact, which depends on the contact impedance. The quality of the measured signals thus depends highly on the contact impedance between the additional or common electrode and the patient.